By Sophie Citro
Kwiaht
The unmistakable, flashy wingtips of the Julia Orangetip once graced our islands every year. Now they are rarely seen. Anthocaris Julia is a species of White butterfly (family Pieridae) that looks like the Island Marble, but is easily distinguishable by its bright coloring. In the islands, Orangetips mainly fly in May, and lay their eggs on Turritis (aka Arabis) glabra, a wild native “Tower Mustard” found on dry hillsides and coastal meadows. Why are Orangetips disappearing? The explanation is simple. Orangetips’ host plant has also been disappearing from the San Juan Islands.
Butterflies previously known as Sara Orangetips are found throughout the Rocky Mountains and the Pacific Coast states. In 2022, they were discovered to be a “species complex” of Anthocaris sara, Anthocaris julia and Anthocaris thoosa. A species complex is a group of closely related species that are often only distinguishable genetically and can sometimes hybridize, making their identification even more difficult. Subsequent analysis of Eurasian Orangetips found that they, too, are a complex of several cryptic species.
Orangetips co-evolved with some toxic plants in the mustard family (the Brassicaceae). Orangetip caterpillars apparently prevent sinalbin, a glucosinolate in the mustards they eat, from turning into toxic “mustard oil” when the leaves are bruised or chewed. At the same time, researchers surmise, sinalbin confers a foul taste on the Orangetip caterpillars that discourages birds from eating them. The orange wingtips of these butterflies may represent a warning to potential predators, as well as an invitation for other Orangetips to mate!
The creamy-white eggs that adult female Orangetips lay on mustards turn a vibrant red after just a few days, warning off other females. This behavior ensures that each caterpillar has a whole mustard plant to itself, and that more of the mustard plants growing in the vicinity are used by Orangetip caterpillars. It also means that the number of Orangetip larvae that can survive each year is strictly limited to the number of mustard host plants that are available.
Orangetips have evolved a clever mechanism to work around this limitation on their abundance. Once the Orangetip larvae have eaten their fill of their mustard plant hosts and are ready to pupate, they can remain dormant (diapause) in their chrysalises for up to three years. If one generation of Orangetip adults falls short, the “backup” pupae from a previous generation that are still in diapause can metamorphose into adults, lay eggs, and save the species from collapse.
To recover the islands’ Julia Orangetips, could we simply plant more of their preferred host plant? Tower Mustard is native to the San Juan Islands, easy to grow, and a part of meadow restoration projects being carried out elsewhere. After all, federal agencies are funding the planting of non-native Brassica rapa, the “Field Mustard” preferred by the Island Marble butterfly subspecies.
It may already be too late to save the islands’ Julia Orangetips, but there are still other regionally rare butterfly species that can be protected, such as the Propertius Duskywing on Mount Young and the Ochre Ringlet at Cattle Point. By protecting native host plant species and keeping the San Juan Islands’ coastal dunes and wildflower meadows intact, we may be able to give the next declining rare butterfly species a happier ending.
